1. Field of the Invention
Aspects of the present invention relate to data modulation, and, more particularly, to a method of code generation and a method and apparatus to allow for code modulation.
2. Description of the Related Art
Run length limited (RLL) codes are some of the most commonly used modulated codes in a data recording system, such as an optical disc drive system. RLL codes limit the run length of a code stream. Here, the term ‘run’ refers to the number of consecutive zeros between ones in a code stream. The RLL codes are defined according to a constraint, RLL(d, k), where d is the minimum number of zeros between ones and k is the maximum number of zeros between ones in a code stream. The value of k is restricted such that at least one transition occurs at intervals of k+1 bits in a modulated non-return-to-zero inverted (NRZI) waveform, allowing the system to recover a timing signal within a time of k+1 bits.
In addition to referring to the minimum number of zeros between ones, the value of d also refers to a minimum time interval between consecutive transitions in the NRZI waveform that is determined to avoid inter-symbol interference (ISI). Further, it is noted that RLL(2, 10) and RLL(1, 7) constraints are widely used in modern optical discs. However, as the bit density on a disc increases, the length of marks recorded on the disc decreases to the point where recording and reproducing data with existing RLL constraint codes may become problematic.
FIGS. 1A and 1B show analog signals detected from a disc on which 8T, 2T, 4T, and 6T waveform segments of RLL(1, 7) codes are recorded.
FIG. 1A shows analog signals detected from a disc on which 8T, 2T, 4T, and 6T waveform segments are recorded within a relatively long mark. As shown in FIG. 1A, even though a relatively long segment (8T) is recorded next to a relatively short segment (2T), since the mark is relatively long, the signal may be well detected.
FIG. 1B shows analog signals detected from a disc on which 8T, 2T, 4T, and 6T waveform segments are recorded within a relatively short mark. As shown in FIG. 1B, the 8T, 2T, 4T, and 6T waveform segments are recorded on the disc in the same order as in FIG. 1A. However, since the mark is relatively short and a short segment (2T) is recorded next to a long segment (8T) within the short mark, the short segment (2T) may not be detected clearly.
The reason for this is that when a recording process is performed on a high density medium, such as a super-resolution near-filed structure (super-RENS) medium, a large amount of energy is applied to record a relatively long waveform segment. The large amount of energy heats the recording medium and affects subsequent recording processes. In particular, when a relatively short waveform segment is recorded immediately following a longer segment within a short mark, the heating of the medium during the recording of the longer segment reduces the clarity and distinction with which the following shorter segment is recorded. Consequently, when a mark is as small as is allowed, recording relatively short segments adjacent to relatively long segments often results in poor detection of the short segment.